PF-4 is a protein-proteoglycan complex released from platelet granules during aggregation which neutralizes heparin. The heparin neutralizing protein subunit can bind heparin and other glycosaminoglycans and has been used as a marker for platelet in vivo release. Experiments are proposed to: 1) purify and characterize the proteoglycan carrier portion of PF-4; 2) study the interaction of PF-4 with glycosaminoglycans and with coagulation proteins like thrombin and antithrombin; 3) extend clinical observations on the use of RIA techniques to measure platelet activation in patients with thromembolism; 4) compare PF-4 turnover with 51Cr survival; 5) study the catabolism and biotransformation of PF-4 following its release. The goal of the studies is to further understanding of the platelet role in hemostasis and to establish non-invasive methods to assess thrombus formation and evaluate drug therapy.